In the conventional electrophotographic photoreceptor inorganic photoconductive compounds such as selenium, zinc oxide and silicon has been widely used as main components, however, these compounds are not satisfactory in view of sensitivity, heat resistance, humidity resistance or durability. Further, some of these compounds are harmful to human body, and have a problem on discarding.
Recently, the studies on organic electrophotographic photoreceptors, which comprise a photoconductive layer containing organic photoconductive compounds, have been eagerly made. Most of organic electrophotographic photoreceptors have a functionally separated structure which consists of a charge generation layer (hereinafter referred to as a CGL) containing a charge generation material such as an azo compound, a phthalocyanine or a multi-condensed ring containing pigment and a charge transport layer (hereinafter referred to as a CTL) containing a charge transport material.
As the charge transport material pyrazolines, hydrazones, triphenylamine compounds and styryl derivatives thereof are well known, however, these compounds are capable of transporting a hole. In a functionally separated type electrophotographic photoreceptor having a CGL as a lower layer and a CTL as an upper layer the surface of the photoreceptor requires to be negatively charged. Therefore, the conventional developer used in inorganic photoreceptors can not be used. Ozone occurred when the electrophotographic photoreceptor was charged by corona-charger is more as compared with positively charged inorganic electrophotographic photoreceptors. A reversely layered electrophotographic photoreceptor having a CTL as a lower layer and a CGL as an upper layer and a single layered electrophotographic photoreceptor containing a charge transport material and a charge generation material in admixture are positively charging, but not satisfactory in view of durability and sensitivity.
In order to solve the above problems, materials capable of transporting an electron are necessary as a charge transport material. As these materials 2,4,6-trinitrofluorenone is known as a charge transport material. However, this compound has poor compatibility with a polymer binder, and has not sufficient properties as a photoconductive layer. Further, this compound causes cancer and can not be used.
Recently, several electron transport materials having a solubilizing group in an electron accepting structure are reported. These materials are described, for example, Japanese Patent O.P.I. Publication Nos. 2-135362/1990, 2-214866/1990, and 3-290666/1991.
The fluorenone compounds disclosed in Japanese Patent O.P.I. Publication No. 5-279582/1993 and "Robunshu, Japan Hard Copy, '92, p. 173" are excellent compounds giving low residual potential and high sensitivity, however, the photoreceptor containing these compounds has problems that residual potential is increased when charge & exposure proceeds on it repeatedly. The quinone compounds disclosed in Japanese Patent O.P.I. Publication No. 1-206349/1989 are excellent electron transport compounds having no plural nitro groups which is said to induce cancer, but have problems that residual potential is high.